1. Field of the Invention
The present invention relates to a method and sensor for gas analysis using a response time constant of the gas sensor.
2. Description of the Prior Art
A gas sensor with a semiconductor element having a selective means for detecting gases is disclosed in U.S. Pat. No. 3,999,122. The semiconductor element is a field effect transistor with a source, a drain, and a channel region extending between the source and drain and reaching to the surface of the semiconductor element. The selective means is a layer of .beta.-carotene covering the channel region at the surface of the semiconductor element. The .beta.-carotene is sensitive to gases and is used in the semiconductor field effect transistor to influence the charge in the channel region and thereby effect a different conductivity behavior of the field effect transistor, and/or to modify the threshold voltage.
Additional gas sensors are also disclosed in U.S. Pat. Nos. 4,020,830; 4,332,658; 4,499,423; and 4,457,161.
Gas sensors for detecting hydrogen present in a hydrogenous compound are known from Appl. Phys. Letters, Vol. 26 (1975), pp. 55-57. The gas sensors include an MOS transistor with a gate electrode of palladium. Palladium, like, for example, rhodium, is a metal that has a catalytic effect for hydrogen and is capable of splitting atomic hydrogen from molecular hydrogen compounds. The atomic hydrogen diffuses through the palladium metal of the gate electrode to the oxide layer of the MOS transistor disposed between the electrode and the semiconductor surface. There, the hydrogen is absorbed to effect the formation of a dipole layer that causes a change in the transistor threshold voltage.
The above-described gas sensor is not useful for gases that are free of hydrogen. In the publications "ESSDERC", Munich, September 1979; "Int. Vac. Conf.", Cannes, September 1980; and "IEEE Transactions", Edition 26 (1979), pp. 390-396 are disclosed gas detectors for other gases, such as carbon monoxide. An MOS transistor with a gate electrode, preferably of palladium, having a plurality of holes therein that reach up to the metal oxide boundary layer, is shown. The use of an NMOS transistor is also considered in this context. Such transistors having a perforate palladium gate have good sensitivity for carbon monoxide and a greatly diminished "cross-sensitivity" for hydrogen. Such cross-sensitivity is sensitivity with respect to another gas in addition to the actual desired sensitivity of the sensor to a first gas. For such known sensors, the change of the threshold voltage is generally linearly dependent on the gas concentration. It is viewed as a disadvantage that the response of such gas sensors is a dynamic process that commences with a certain chronological delay in response to the influence of the gas, for example, carbon monoxide. In the aforementioned U.S. Pat. No. 4,499,423, electronic measures are employed to compensate for this disadvantageous time dependency. Both the delay time constant and the quantitative sensitivity of the known sensors are temperature dependent.
The cross-sensitivity of a sensor can be diminished by auxiliary measures. For example, the cross-sensitivity for hydrogen of the above-described carbon monoxide sensor can be reduced by more than an order of magnitude by applying a special protective layer.
While palladium has been used in gas sensors as described above, rhodium, platinum, and nickel are also known hydrogen-permeable substances. Silver, on the other hand, has a pronounced selective permeability for oxygen. Tin anhydride is used in a gas sensor, manufactured by the Figaro Company, which is sensitive to combustible and toxic gases. The sensitivity of the tin anhydride is based on a resistance change in the tin anhydride that has been rendered conductive.
Gas sensors that work on the principle of calorimetry are known by the name "Pellistor". A pellistor is composed of two platinum resistance wires into each of which a porous ceramic tablet is sintered. A catalyst is applied to one of the two ceramic tablets. A measurable increase in resistance is detectable at the platinum resistance wire having the ceramic table coated with the catalyst during catalytic burning of the gas to be detected. The increased resistance is sensed by inserting the two platinum resistance wires into a bridge circuit to compare the resistance of the first platinum wire to the second platinum wire.
Calorimetric effects in conjunction with catalysts are also known in the prior art. These include the combustion of hydrogen at a platinum catalyst, which produces NO from NH.sub.3 with platinum or platinum-rhodium as a catalyst at 200.degree.-250.degree. C. Also produced is NO.sub.2 from NO with a catalyst of Al.sub.2 O.sub.3 -SiO.sub.2 gel at 100.degree. C. upon the addition of a corresponding quantity of oxygen. SO.sub.2 can be oxidized with oxygen to form SO.sub.3 at elevated temperatures with the assistance of a platinum catalyst, and with the assistance of a catalyst of Fe.sub.2 O.sub.3 and with V.sub.2 O.sub.5 as a catalyst. With the assistance of palladium, CO can be oxidized to form CO.sub.2 at temperatures of or above 150.degree. C. With the assistance of a silver catalyst, methanol can be oxidized to form HCHO at 200.degree.-400.degree. C. Further catalytic processes are known from "Gmelins Handbuch der organischen Chemie", from Winnacker-Kuchler, "Chemische Technologie", from Ullmans "Enzyklopadie der technischen Chemie" and from Reich, "Thermodynamik". Further publications that relate to semiconductor sensors are found in: IEEE Transactions on Biomedical Engineering, Vol. BME 19 (1972), pp. 342-351; IEEE Transactions on Biomedical Engineering, Vol. BME 19 (1972), pp. 70-71; Umschau (1970), p. 651; Umschau (1969), p. 348; German Pat. No. 1,090,002; and U.S. Pat. No. 3,865,550.
Zeolites are known as molecular sieves and are used for their selective effect on gases. Such molecular sieves have the property of allowing molecules of specific size values and below to pass therethrough and blocking the passage of larger molecules. Numerous examples of usable zeolites are known from Grubner et al., "Molekularsiebe", VEB Dt. Verl. d.Wissensch., Berlin (1968).